(1) Field of the Invention
The present invention relates to a heat-sensitive black recording material. More particularly, the present invention relates to a heat-sensitive black recording material which is capable of forming a substantially purely black image not only at a high density but also at a low density with an excellent gradation.
(2) Description of the Prior Art
Conventional heat-sensitive black recording materials are roughly divided into two types. Recording materials of one type comprise a plurality of leuco dyes forming colors complementary to each other by reaction with an acidic substance and recording materials comprising a fluoran type leuco dye for formation of a black color alone. In the heat-sensitive black recording materials of the former type, images having a hue close to a purely black color can be formed at a sufficiently high temperature, but images in which an inherent hue of any of the leuco dyes is strong are formed at a relatively low temperature. Namely, a problem of so-called color shearing arises at a relatively low temperature.
Fluoran type leuco dyes capable of singly forming a black color have recently been developed, and they have been used for commercial products but some problems are left unsolved. More specifically, these fluoran type leuco dyes for forming a black color are relatively low in sensitivity to heat, and therefore, they should be used in combination with a sensitizer such as a fatty acid amide. Moreover, these fluoran type leuco dyes are defective in that when the image density is relatively low, the hue of the formed image tends to become greenish, and this tendency is especially conspicuous when the sensitizing agent is used in combination. Accordingly, in heat-sensitive black recording materials of this type, formation of an image having a good gradation, especially a halftone image, is difficult. Moreover, it is difficult to perform recording at a relatively low temperature.